In the processing of semiconductor wafers, the semiconductor surface preparation prior to various processing steps such as oxidation, deposition or growth processes, has become one of the most critical issues. With the rapid approach of subhalf micron design rules, very small particles with submicron dimensions and low levels of metal impurities (-10.sup.10 atoms/cm.sup.2) can have a detrimental effect on process yields. A commonly used technique to reduce foreign particulate matter contamination level on semiconductor surfaces, is the immersion of wafers in liquids such as chemical solutions. The semiconductor wafers are usually placed into a tank or vessel or wet bench, which holds a liquid for cleaning or etching. Such tank or vessel or wet bench is installed in a clean room environment. Often a wet bench contains several vessels or tanks.
Semiconductor wafers or substrates can be immersed into the liquid from the upper part of the tank. Another way to put the substrates in contact with the liquid is to use a door or an entrance at the side wall of the tank and to place a batch of wafers into the tank. After the door is closed and sealed, liquid is flowing into the tank, again to make contact between the cleaning or etching liquid and the substrates. In both approaches, one can switch from one liquid to the other liquid in the same tank. This method is commonly denominated as a single tank processor. Variations of these two techniques are also used for single wafer cleaning or etching purposes. In such a case, the wafer can be placed into a small vessel, which can be larger than the wafer. In some cases, the wafers are immersed into the liquid; in other cases, the wafers are loaded into an empty vessel and the liquid is flowing into the vessel.
A particular apparatus used after cleaning or etching of semiconductor wafers is disclosed in EP 0 385 536, the teaching of which is incorporated herein by reference.
EP-A-407-044 discloses a system having two rooms, each containing a liquid. The system can be used in a semiconductor wafer manufacturing line and allows a wafer to be transported from one room of the system to another room of the system without an intermixing of the liquids occurring. The level of the liquids in the two rooms is adapted as to adjust the respective pressures of the liquids and an elaborate shutter device is provided in the system for opening and closing an opening between the two rooms without causing intermixing through flow and turbulence of the liquids.
A first drawback of the above-mentioned tools and methods is the footprint necessary, i.e., the surface area within a clean room occupied by the tank or vessel. Above-mentioned tools making use of a tank or vessel are big and occupy a large area within a clean room. It is to be understood that clean room area is very expensive due to the costs of maintaining an ultra-clean atmosphere.
Another drawback is the throughput of wafers. When, for example, a single tank processor is used, switching from one liquid to another liquid takes time and the system can have a lower throughput compared to a classical wet bench approach.
Another drawback of the prior art cleaning or etching tools and methods is that a non-uniformity in the cleaning or etching process occurs remains. This is to be understood as follows: when a wafer is dipped in a tank, the bottom part of the wafer is always remaining some longer in the cleaning or etching liquid than the upper part of the wafer. Therefore, the contact-time of several parts of the wafer is not identical.
A further drawback of the prior art cleaning or etching tools and methods is that a drying and/or rinsing step is required after a cleaning or etching step. This drying and/or rinsing step takes extra time and therefore increases the processing time of wafers.
Aim of the Invention
It is an aim of the present invention to provide an apparatus and a method for wet cleaning or etching a flat substrate wherein the above-mentioned drawbacks of the prior art tools and methods have less impact. The apparatus and the method of the present invention allow a high throughput of flat substrates and a high cycle time, allow a more uniform contact time for several parts of one wafer, allow the development of manufacturing systems with a small footprint and allow flat substrate manufacturing with the wet cleaning or etching step and the required drying procedure being executed in one processing step.